Manufacture of cold-cathode electric discharge devices



March 4, 1958 I J. 5. MILLER 2,825,619

MANUFACTURE OF COLD-CATHODE ELECTRIC DISCHARGE DEVICES Filed Jan. 3, 1955 FIG.2 MG 3,

FITTORNEY United States Patent MANUFACTURE OF COLD-CATHODE ELECTRIC DISCHARGE DEVICES James Samuel Miller, Harrow, England, assignor to The General Electric Company Limited, London, England Application January 3, 1955, Serial No. 479,634

5 Claims. (Cl. 316-26) This invention relates to the manufacture of coldcathode gas-filled electric discharge devices of the type wherein the cathode consists of a metal support carrying on at least one surface an electron emissive coating containing one or more metal oxides of lower work function than the metal of the support.

More particularly, though not exclusively, the invention relates to glow discharge devices of this type such as are employed in electrical communication systems, for example telephone circuits, for switching operations and the like.

The metal oxides most usually employed in devices of the type specified are the oxides of the alkaline earth metals, although other oxides, for example thoria, are sometimes used. In order that the oxide coatings may have a suitably low work function, it is necessary to activate them after they have been formed, that is to say to subject them to a treatment which will reduce some of the oxide to free metal, and one known way of eflecting this electrically is to subject the coating to the action of a high frequency spark discharge applied between the anode and cathode of the device; this treatment has been effected with the device containing its normal gas filling and continued until the high frequency spark discharge playing over the oxide-coated surface of the cathode has activated the whole of the surface. It has been found, however, that the thickness of the oxide coatings that can be satisfactorily activated by this process is limited, usually to that corresponding to a coating weight of less than 1 milligram per square centimetre. device of the type specified depends, however, on the amount of activating material present, and hence on the thickness of the coating, and the object of this invention is to provide an electrical method of activating the oxide coating which is less subject to this limitation on the thickness of the coating.

The invention depends on a phenomenon which we have observed when a gas-filled discharge device having an unactivated oxide-coated cathode, and a filling pressure of only a few millimetres mercury, is operated with a unidirectional applied voltage between the cathode and anode and with the cathode negative with respect to the anode. As the voltage is increased from zero, a discharge through the device commences when the voltage reaches a certain critical value and is characterised by a bright spark which forms on the surface of the cathode and rapidly moves from point to point over the cathode surface. This condition of the discharge is, however, very unstable and usually the current either falls to a value at which an abnormal glow discharge forms on those parts of the cathode surface already activated or increases to a value where a stationary thermal are appears hetween the anode and the cathode, depending on the particular circuit arrangement employed. We have now found that this spark condition of the discharge can be utilised for efiecting a satisfactory activation of the whole of the oxide coating eveu'with thicknesses sorts- The 'life of av ice spending to a coating weight in excess of 1 milligram per square centimetre.

According to the invention in the manufacture of an electric discharge device of the type specified the oxide coating of the cathode is activated by a method which comprises connecting the oxide-coated cathode to the negative terminal of a direct current supply, connecting another electrode of the device to the positive terminal of the supply, and passing an electric current between the cathode and said other electrode whilst the envelope of the device is filled with an ionisable gas filling at a low pressure, said current and pressure being such that there is obtained a highly mobile spark discharge which moves rapidly from point to point over the surface of the coating and activates the coating, the value of the discharge current being controlled during the activation process so as to lie between limits at which the discharge is maintained in said highly mobile spark condition until the activation of the coating by said method has been completed.

The limiting values of the current for the activating process will, of course, depend upon the electrode geometry and the actual gas filling employed, and may readily be determined by trial for any particular form of device as required.

We have found that the gas filling for the activating process preferably lies between 5 and 15 millimetres mercury, which is in general much lower than the final gas filling pressure of the device.

When the whole of the cathode coating has been activated by the method according to the invention, the highly mobile spark discharge condition ceases, and the disrent and circuit arrangement used, and this change givesan indication that the activation process has been completed.

The effective resistance of a device of the type specified whose cathode is being activated in accordance with the invention will vary as the process proceeds, and the control of the current through the device within the required limits, so as to maintain the discharge in the highly mobile spark condition, may either be controlled manually, for example by means of a variable resistance in series with the device, or automatically, as desired.

For controlling the current automatically the device may be connected in series with a pentode-like thermionic valve, that is to say a valve which can be arranged to operate on a substantially flat part of an anode current/anode-cathode voltage characteristic such that over a considerable range of voltages applied between its anode,

and cathode, the anode current varies only slightly; some forms of tetrode as well as pentodes and valves having more than five electrodes are of this kind. The valve is arranged to operate on a fiat part of its characteristic as aforesaid with the anode current adjusted so that the current through the device lies within the range of values required for the activating process to he carried out in accordance with the invention, the current through the device during the activating process being maintained at a substantially constant value (that is to say within a relatively small range of variation from a mean value, for example plus or minus 5 milliamps) within said limits despite changes in the eflective resistance of the device and the corresponding changes in the anode-cathode voltage of the valve.

Thus the device may form part of the anode load of .a tetrode valve the control grid and cathode of which are connected together, and the screen grid of Whichis arranged to be maintained at a potential relative to the cathode which is controllable for adjusting the anode current to a required value, the anode current being a'd processproceeds, thereby justedbforethe commencement of the activation process to 'a value which is held *substantiallyconstaht throughout the process and which is such that during the process the discharge current through the device lies between said limits 'andismaintain'ed substantially 'n'stant uiitilthe process is completed. I p i i v p By employing 'a circuit arrangement such that when the "oxide coating off't'he devicehasi been completely activated theincrease 'in efiective resis'tance "of the ?device results in the 'valve no longer operatingjwithin the said range 'of "anoileca'tho'de voltages witliin'v/hich there are "only slig ht "variations in the'anode curfentfthe decrease in current through the fdevice, associated with transition for example by the change in the reading ofamilliammeter'connec'ted'in series' 'with theiievice. 7 V i s The inethod of activation in accordance-With the invention is especially suitabl'e' for activating cathode coatings con'si'sti'ng'of-one or-n1ore-of the all alineearth metal oxides, and satisfactory activations of coatings of this kind having a coating weight are milligrams per square centimetreihavesuccessfullybeen carried out. 1

determined by trial for ny particular form of device as required. it a The method of activating the cathode in accordance withthe invention may be employed to advantage in 'the manufacture of devices of the type specified having, in addition to the cathodetand anode, a third elec trode, more often referredq to as the trigger electrode,

7 located in line with and betweenathem. Owing to the to=theabnorrnalglow regime, may be arranged togivefa further indication that-the activating process iscoh pleted, '15

"Devices having a relatively long operating-life can 7 thereby be constructed iby employingan 'activatingprocess in accordance with the invention, such devices -bei-ng for example as switches in telephone circuits. a

In devices in which the cathode coating'c'onsistsofone or more of the alkalincaearthmetalloxides, the unact-ivated oxide coating has a white powdcryja'ppearance, 'butat every point of contact of the rapidly moving sspar k with especially suitable for, use inapparatuswhich" has to be operated for long' :periods with little 'or no attention',

the coating the colour of the coatin'g'changes toithat 'o'f'a very dark .brown,;the movement of the spark over the surface thus producinga series of dark specks which-grad ually -spread to .form ai-uniformly dark surface as the V givingia further indication when the processtis'completed. V If; desired the activation process according to' the invention, ortprimary process as it will hereinafter be referred' to, may be followed by'a secondary activation process forbringing the cathode coating of the device to an even higher-state of activation than ,may zbe obtained byssubjecting it to said primaryprocess alone", 7 In this secondarvprocessa series of short duration sparkJdischarges,..having-peak currents'of several amperes, are, arrangedttoltake place between the cathode and at least one other, electrode of the device, with'the cathodc' acting as the negative electrode, whilst the :envelope of the device is filled with an ionisablegas-filling at, or

7 approximately at, the :fin'al sgasfilling pressure of the, device. I

' This double activatiomprocessactivates the oxide cathode'coating to an eventhigher degreet than does the-prifor coatings having a coating weight of over ,1 milligram per square centimeten since the primary activation of the cathode fuses the material of the coating and renders it able to Withstand the high current spark discharges of 'the secondary activation process. a

The secondary'activation process isprefr'dbly arried' but after the envelope ofthe device has been sealed off, the series of spark discharges conveniently beingpro= gduced by repeatedly discharging'a condenserthrough the device as in some known methods of cathode activation; 1 During the Secondaryactivation process the appearance ""Thus referring to Figure 1 7 shown comprisesia-flat' rectangular nickel sheet :10 milli metres long by '8qmillimetresiwide by-0.2 millimetres fact that the trigger electrode is u'sually situated only a relatively short fiistance fromthe' coated surface of the a cathode, theprimaryfact'ivation is v preferably carried outby passing a controlled:currerit betw'een the anode -an'd the cathode, thus enabling-the activation .to' be effected over a larger area than if the controlled current was passed between the triggereelectrofle an'd'the cathode. However in such a process part7 ofv the cathode surface is shielded from the anode by the trigger electrode, thereby regions-5r the coating, are 'thenfbrou'ght to a'n'e'ven higher state'of 'activ ati'on "as the process proceeds.

The method of activation according to the invention may also-useful1y lbe employed foractivatihg'the cathode; of devices'o'f the type specified having a trigger "electrode, located adjacent to "an edge of thetc'atho'de', cs1

ing in the region of the trig r discharg penises-thee V M in operation or the "(16916618 regs -1ikely'to uncoated surface of the cathode.

" The manufacture or "two cold-cathode electric .dis-

charge devices -of the type specifiedin accordance with the invention will nowfbe' described by way-of example 7 4 'withreference to Figures 1 10 4 or the accompanying schematic drawinggin :WhiChfi p 7 v e Figure 1 'represents a perspective view Of'fljiWO elec trode device, t

'Figurel' shows a particularcir'cuit arrangement which 7 may be employed for --autornatic'ally maintaining'the discharge current through the device at a substan'tially'; con- -stant va1ue as theprim'ary activation process is carried a Figure 3 represents a perspective view ofa-thre'e electrodetdevice,tand' V V a V V Figure 4" shows a particular circu'it arrangement :for

cathode of the device;

the cathode 1 of thedevice thick, on onezsurface ofywhich isvprovidcd the electron emissiveoxide coatingsconsisting offia combination of barium, strontium and calcium oxides. To th'e other, uncoated,-1surface of the cathode is welded a nickelisupporting svire-2, thegvvir e beingilocated centrallyib'etween' of alkaline earth oxide' coatings (which'arel the coatings most commonly used) undergoes a change froma dull;

V l dark brown colour, resulting from theprimary activation 7 process,to asmooth shiny metallicappearance thetdegree. or activation effected being indicated by theamount of 'changeproducedin the colour ofthe coating The duration of the process-will depend on the extentto which the coating is required to be activated, and on the-frequency hr t net e spar -di har s: anqmav readily. be I wards towardsatheraxis toft 'the idisc. V, V a Also sealed intowthe 'di'sc :3 so that it extends pcrpenthe two longer edges of zthe fcathode and projecting,

thereof, ith ewouterae'nd of the wire being sealed into a pressed tglass zdiscafi which constitutes the base ofxthe device, with .theicoated surfacehoftthe cathode facingsindicularly therefrom in ;the same a direction as the support a a a fl ihttg increasing the state "of activation of fthefcafl ode jcoat and tfieiriggenleen aqa form :on "the carrying out the secondary a'ctivation process '-for the'i asaaeie face of the cathode, is an additional nickel rod which constitutes the anode 4 of the device, the distance between the anode 4 and the cathode surface being 8 millimetres and the length of the anode being such that its free end is approximately level with the top end of the cathode 1.

The cathode support wire 2 and the anode 4 are continued within the disc 3 by lengths of copper-clad nickel wire for satisfactory sealing into the disc, and these lengths of wire are continued on the opposite side of the disc by nickel-iron-chromium rods 5 which are embedded at one end in the disc and constitute the terminal pins of the device.

The anode and cathode are enclosed within a substantially tubular glass discharge envelope 6 into one end of which is sealed, for example by drop sealing, the pressed glass disc 3, and which is closed at the other end by a tipped off pumping stem 7, the tube containing, after manufacture, a gas filling consisting of a mixture of, by volume, 80% neon and 20% argon at a total pressure of 40 millimetres of mercury.

During the manufacture of the device the nickel sheet which forms the cathode 1 is first welded to the nickel support wire 2, and is then washed in carbon tetrachloride followed by heating to a temperature of 1000 C. in an atmosphere of hydrogen for 15 minutes in order to cleanse the surface of the cathode and to remove surface grease.

The surface of the cathode to which the support wire 2 is welded, hereinafter referred to as the rear surface of the cathode, is then coated with aluminium paint in order to prevent glow discharges being formed on that part of the cathode in operation of the device, and the cathode is then reheated in an atmosphere of hydrogen to 925 C. for 15 minutes, there being a slight amount of oxygen present which is sufiicient to oxidise the aluminium so as to form a sintered coating of aluminium oxide on that surface of the nickel.

The oxide coating, which consists of a combination of barium, strontium and calcium oxides, is formed on the front surface of the nickel sheet by spraying it in known manner with a suspension of the appropriate carbonates in a suitable binder to form a coating of 3.3 milligrams of the carbonate per square centimetre, the carbonate coating being decomposed after the cathode has been mounted within the device and whilst the envelope of the assembled device is connected to the exhausting pump, by means of eddy current heating to 1100 C. so as to form a triple oxide layer on the cathode.

The activation of the cathode coating using only the primary process in accordance with the invention will now be described. For this purpose the envelope of the device is filled with a mixture of, by volume, 80% neon and 20% argon at a pressure of 5 to 10 millimetres of mercury and a steady D. C. potential is applied between the cathode and the anode of the device, with the cathode connected to the negative terminal of the supply, of such a value and so controlled as to obtain a substantially constant arcing current of 100 milliamps between the anode and cathode, causing an intense bluishwhite highly mobile spark to form on the cathode surface, the spark moving from point to point on the surface with great rapidity.

Before the activating process the triple oxide coating on the cathode has a white powdery appearance, but as the spark moves rapidly over the surface, at every point of contact the colour of I the coating is transformed to a very dark brown, the random movement of the spark thus producing a series of very dark specks which spread over the surface as the operation proceeds, until the entire coating has a uniformly dark brown appearance and none of the original white oxide remains. The activation of the cathode coating by the mobile spark process is then complete, the time taken from the commencement of the sparking process to its completion being of the order of one minute for a device substantially as de scribed.

For ageing the device the envelope is then reevacuated and filled with a mixture of, by volume, neon and 20% argon at a pressure of 10 millimetres of mercury, and a direct current of 20 milliamps passed between the anode and cathode, with the cathode as the negative electrode, for five minutes. The pressure of the gas filling is then increased to 25 millimetres of mercury and the current is stepped up to 30 milliamps for another five minutes, the gas pressure finally being increased to 40 millimetres of mercury and a current of 40 milliamps being passed for a further period of five minutes.

The envelope of the device with the gas filling consisting of, by volume, 80% neon and 20% argon at a total pressure of 40 millimetres of mercury is then sealed off, the ageing process then being completed by overrunnning with a current of 30 milliamps for several hours.

To maintain the current through the device automatically at the required value during the activating process a circuit arrangement as shown in Figure 2 is employed. In this arrangement there is included a tetrode valve V1, the control grid 8 and the indirectly heated cathode 9 of which are connected to earth, and the anode 10 of which is connected through a resistor R1 to the positive supply terminal 11 of a D. C. supply of 250 volts, the negative terminal 18 of the supply being earthed.

The anode 10 of the valve V1 is also connected to one contact terminal 12 to which the cathode 1 of the cold cathode discharge device 19 is arranged to be connected during the activating process, another contact terminal 13, to which the anode 4 of the discharge device is arranged tobe connected, being connected to the positive D. C. supply terminal 11 through a milliammeter mA.

Agas-filled voltage stabilising valve V2 has its cathode 14 connected to earth and its control electrode 15 connected to the positive D. C. supply terminal 11 through a resistor R2, the anode 16 being connected to the positive supply terminal through a resistor R3, and to the screen grid 17 of the valve V1 through an adjustable resistor R4 for supplying an adjustable but stabilised voltage to the grid 17 for controlling the current through the valve V1.

The values of the circuit parameters are chosen so that with the device connected to the contact terminals 12, 13 as hereinbefore described the valve V1 operates substantially on the flat part of its anode current/anode-cathode voltage characteristic so that although there may be considerable changes in the value of the anode load, and thereby in the voltage drop across the load, the resulting variations in current through the valve and hence the load are extremely small.

For carrying out the activation process a dummy load resistance having a value approximately the same as the. resistance of the discharge device whose cathode is to be activated is first connected between the contacts12, 13 with the device left disconnected and the valve current adjusted to a value of milliamps by means of the variable resistor R4. The dummy load resistance isthen removed and replaced by the discharge device, the cathode 1 of the device being connected to the contact terminal 12 and the anode 4 of the device being connected to the contact 13. The activation of the cathode then proceeds, and is characterised by an intense bluish-white spark rapidly moving over the surface of the cathode 1 of the device as hereinbefore described, the valve V1 maintaining'the current throughthe device at approxi-, mately 100 milliamps despite changes in the resistance of the tube as the cathode is activated. However when the activation process is complete the voltage drop across the device is such that the valve V1 no longer operates on the fiat portion of its voltage/current characteristicand the current through the device drops considerably from.

- of the' device.

ink-ori inal v'alue; thefeby giviiig mindic fienj-ir; seattion to the changein colour ot the cathode coatififgjof therdevice; {that' the aqtivatien pr'oces"sl1'a's-beei1' c'o'rripfletied *Such an indication -may be desirable,'-'for example,

V where the devices are beingmanufactured by mass prodttction 1 methods "in order to ensure that; the {cathode "of each device is completely activated andal's joto reduce the possibility of a de'vice being seriously" over-run when t-h'e activatio'n processhas been 'completedywhich latter might result in a substantial "reduction in the operational life Typical circuit components for maintaininga current. of' 100 milliamps through a discharge deviceconstructed asdescribedby way of-example are as follows:

- .Te'trode valve'V1 "CV 1075; 7

Voltage stabilising valve V2.. .CV 395. 1 Resistor R1 0.1 megohm. I Resistor R2 0.1 megohm; Resistor R3 1500 ohms.

Resistor R'4 e Maximum value 22,-

' 000 ohms.

For a device as described thecurrent during the activation process should lie between a minimum value of ap- V proximately 75 milliamps; (below which 'the mobile bluish-white spark discharge will be replaced byan ab normal glow =discharge on' the parts of the cathode coating which.have already been activated) anda maximum 1 value of about 120 milliamps ,(above' which the discharge will changeinto a :stationary arc discharge having a reddish colour insteadof the bluish-white colour :o'fthe rapid- 1y moving spark discharge). By arranging that the discharge current lies in the region of 100 milliamps any variations either sidefof. this figure whichmi'ght occur withinthesei limitslwill n tEIesuIt-in the interruption-of the activating process.

For-devices comprising-a cathode of :a different "size-or configuration," or different 'gas filling, the above limits will,

. 25 which -are sealed at their -'opposite ends into -a pressed glass .disc. 26 whi'ch lcon'stitutes the base of the device.

'For. supportingtthe cathode the two support-wires 525*are welded to opposite ends of -a nickelmounting wit-e27 which isitself welded'to the uncoated' surface of the oath: ode along one of the shorter edges thereof, thnds of1the wire; 'to which the support-wires are 'welded jprojecting slightly :beyond the'cathodmand the cathode is mounted withiits'plane perpendicular to the glass disc 26' andwith it'scoated surf ace 28 facinginwards towards the diseaxis.

The anode 22 consists-6f a'straight nickel'wire itself sealediatone endinto-the g'lass disc 26 j so that ,itproje'cts assesx5 'el'ectrode' 23. V r v :The-cathode and trigger-elctrodesupport' :5 m 29 respe'ctivel'y, andf'the anode wire- 22 are continued within the glass disc' 26 bydengueetcop er-eiaapicker wire for satisfactory sealinginto the disc, and these-lengths of 'wire are'continued on the opposite side of 'the disc by attheir opp osite efidstdkhe bent' eridsfib manate;-

ni ckel-iron-chroriiium rods 31 whichareembedd'ed at .eifdfin "the disc and which con stitute*theterrninal pins 7 of the device.

T he ionisable g' a's filling of the 'deviceconsists, after perpendicularly therefrom and positioned so'that'it is immediatel-y opposite the*centreof-the coated surface 28 of thecathode .21',-the anode-cathode spacing being :8millimetres. The trigger electrode'23 also consists of a length of nickel wire- 05 millimetre diameter and about 8 bentat rightfangles-away from the cathode so thatthey extend perpendicularly thereto. The spacing of the said 7 centrepart 'ofithe trigg er electrode123,which isapproxi aielya millim tre nj sn th; 1 1 -1 m ll iii 'f j fii c surface or the 'ca'thode; and; it is mounted'wi its centre flp'oin't immediately "opposite .theicentre of'the cathode, means} cf two jsupport' wires }29 -which are each sealed at one'entl into thepressed glass'disc 26 and are'sp'otwelded r 40' The cold cathode-device shown in Figure 3 consists'of 25% 'argon at a total pressure of 40 millimetres of mercury. v V H I I Durin g the'ma'nufacmrefofthe deviceithe'nickel jsheet irhich'forms'the cathodej'll isiirst'wel'ded to the mQLlnt-f ing wire '27 andisfthen; cleaned infajsimilar 'manneritothe similarlyfprovided on the 'rearj'surface with fa c'oati'ng'i'of aluminium paint'andion the frontjfsurface with a "coating of barium, strontium and calcium oxides,"thellattericbat ing; .bei ng formed by decomposition of the appropriate, carbonates 'as'alr'e'a'dy described; y "The pride coating is then "activated by 'theQprirnary activation process, for which pui'pose "the envelope is fille'diwith a mixture; byvolum'e; of %.fneon and 25% argon 'at afillingpres'siire of 5 'to 10 millimetres (5r flier-g cury, asteady D. C. gpotentialis then applied between the. cathode" andfthe anode offthejdevice; with the cathode negative'jv'vithresp'ect 'to'th'e-anode, offsuch a value as to obtain" a 'steady arcing current whichiis .c'cintrolle'd at same; .of 7100 inilliamps, this 'arcing current causing" an intn s'ebluish-white highly mobile spar ktoform' on the t cathddesii'rface anditomove from pointtopointover the 7 surface with great rapidity; for automaticallygcontrolling' armament at therequired value during this activation 7' processfthe circuit arrangement described iby way of example with reference to Figure2 may conveniently-be employed.

{During this primary activation process the white triple oxide is gradually changed to-a very dark brown colour:

as the spark moves rapidlyoverthe cathode surface, and

atthe completion of the process substantially the whole of the coating, apart from the part;of;thej coating shielded TQ11.- h? amide h t i r. relecttoderhas n tmlv dark, 9Wn.: Pnr a9:an n e of t ew t o de cma s; The, hie ed p t o heso s asarslight y lighter appearance :showing that it has 'not -been as fully a m d infi at e is iii e r tf the-c i mixture by volume, of 75% neon and, 25% argon at a filling pressure of 1'0 millimeters ofi mercury, a direct q r ntg q igz ii m s cinep s d bet e n e anode and sat sdsm tths th de. sth ehi -el c d i 7 e-fla e ?I-hQPtfiS q p fth v 'sifi s s-th n 7 i c s d 5. m l m tr sio mercury-an egc e 't isstepped up todQ milliamps for another-five minutes the g smre ure gfina b in inc sed a; mi met es i msreat aniw umm 0 m l mp ngra iu hse er Qffiwmmute -1 c 1 Thel envel ing, by'vdliime, of 75% neon and25% argon, at -a total @Of t 'd se'wit riheis i ll n isti l -P e ur 9M0 mi mctrestof me cur t ms al I 65 millimetres *long,*the centre par tof which lies parallel "to V thecathodeand-theglassfdisc 26, and the two ends being aslwill-nowbe described.{-

Th ath d st sny ubi q ii to thesc o a y a tivation process-in accor ance with theipresent invention,

Proc s. i' -a h wn h mati a the resistor R4 and capacitor C, and the other spark gap electrode 35 is connected through a resistor R5 to the opposite side of the capacitor C and to the cathode 21 of the discharge device. The trigger electrode 23 is connected directly to the spark gap electrode 35, and the anode 22 of the device is left disconnected. A bleeder resistor R6 having a very high resistance is shunted directly across the capacitor C in order to allow any charge on the capacitor to leak away after the process has been completed, and a switch 36 is connected in the positive lead between the positive input terminal 32 and the resistor R4.

On switching on, the voltage across the capacitor C rises as the capacitor charges, the voltage across the resistor R5, and hence between the trigger electrode 23 and the cathode 21 of the discharge device, remaining at zero whilst no discharge occurs between the spark gap electrodes. However when the voltage across the capacitor C reaches a value sufficient to cause a spark to pass across the said gap, substantially the whole of the voltage across the capacitor is applied across the resistor R5 and hence across the trigger-cathode gap of the device. This voltage is suflicient to cause a spark discharge to occur between the trigger electrode 23 and the cathode 21, and the capacitor discharges through the spark gap and the device. As the voltage across the capacitor drops below a certain value the discharge ceases, the resistance of the path through the spark gap and the device regain their original value and the capacitor commences to recharge, the operation then automatically being repeated. The charging and discharging of the capacitor occurs with great rapidity, the repetition rate and the amount of charge- Capictor f 0.1 Resistor R4 nhms 470,000 Resistor R5 m gohms-.. 4.7 Resistor R6 o..- 10

Distance between spark gap electrodes 0.05-0.1 mm.

With such a circuit arrangement a satisfactory activation of the cathode is obtained by subjecting it to the spark discharges of the secondary activation process for a period of about 2 to 5 minutes.

The secondary activation process in accordance with the invention may also be used for activating the cathode in devices of the kind specified employing only a cathode and an anode, for example as shown in Figure l, the spark discharges of the secondary activation process then being arranged to take place between the anode and the cathode, and the process may also be used in devices having one or more trigger electrodes which are not located in line with the cathode and anode. In one example of the latter kind of device the anode and cathode are formed substantially as in the device shown in Figure 3 of the present drawings, and the device is provided with two trigger electrodes, each consisting of a nickel wire arranged parallel to the anode, and located about 1 millimetre from opposite edges of the cathode, slightly nearer to the anode than the smooth continuation of the coated cathode surface but not in line with the anode and the cathode.

When activating the cathode of such a device by a secndary activation process employing a circuit arrangement-substantially as shown in Figure 4 of the present drawings both trigger electrodes are vsimultaneously con-. nected to the junction of the spark gap electrode 35 and the resistor R5, and regions of the coating extending along the edges of the cathode adjacent to the trigger electrodes are subjected to the high current spark discharges for a period of about 2 to 5 minutes. The increased activation of the coating in this region reduces the possibility of the auxiliary discharges, produced between the trigger electrodes and the cathode in operation of the device, from forming on the rear surface of the cathode, even when the trigger electrodes are almost in alignment with the coated surface of the cathode.

It will be appreciated that the double activation process may also be used for activating the cathodes of devices of the type specified other than those hereinbefore particularly described; the values of the components used in the circuit arrangement employed for producing the spark discharges of the secondary activation process depend upon the electrode geometry of the device whose cathode is to be activated in any particular case, and may easily be obtained by calculation or trial.

I claim:

1. The manufacture of a cold-cathode gas-filled electric discharge device of the type in which the cathode consists of a metal support carrying on at least one surface anelectron emissive coating containing one or more metal oxides of lower work function than the metal of the support, wherein the oxide coating of the cathode is activated by a method which comprises connecting the oxide-coated cathode to the negative terminal ofa direct current supply, connecting another electrode of the device to the positive terminal of the supply, and passing an electric current between the cathode and said other electrode whilst the envelope of the device is filled with an ionisable gas filling at a pressure below the final gas filling pressure of the device, said current and pressure being such that there is obtained a highly mobile spark discharge which moves rapidly from point to point over the surface of the oxide coating and activates the coating, the value of the discharge current being con trolled during the activation processso as to lie between limits at which the discharge is maintained in said highly mobile spark condition until the activation of the coating by said method has been completed.

2. The manufacture of a cold-cathode gas-filled elec tric discharge device of the type in which the cathode consists of a metal support carrying on at least one surface an electron emissive coating containing one or more metal oxides of lower work function than the metal of the support, wherein the oxide coating of the cathode is activated by a method which comprises connecting the oxide-coated cathode to the negative terminal of a direct current supply, connecting another electrode of the device to the positive terminal of the supply, and passing an electric current between the cathode and said other electrode whilst the envelope of the device is filled with an ionisable gas filling at a pressure of between 5 and 15 millimetres of mercury, said current being such that there is obtained a highly mobile spark discharge which moves rapidly from point to point over the surface of the oxide coating and activates the coating, the value of the discharge current being controlled during the activation process so as to lie between limits at which the discharge is maintained in said highly mobile spark condition until the activation of the coating by said method has been completed.

3. The manufacture of a cold-cathode gas-filled electric discharge device of the type in which the cathode consists of a metal support carrying on at least one surface an electron emissive coating containing one or more alkaline earth metal oxides of lower work function than the metal of the support, wherein the oxide coating of the cathode is activated by a method which comprises connecting the oxide-coated cathode to the negative terminal of a direct current supply, connecting another theactivation process so asto liebetw'een limits electrode of the device to t'he positive -"t'erin i1ial{ of "the supply, and passing 'an electric current between the the activation "of the coating by said "method has been completed; i 7 I 7 r 4. The manufacture of a'cold-cathode gasfilled electric discharge deviceiof'the type in which the "cathode con- 'tiv'e electrode, whilst" the envelope'of with 'an'ionisabl e gas filling-at approximately the final gas filling pressure of the device. I

5; The maniifacture of a cold-cathode gas fille'd elec-: tric discharge device'of the type provided with an anode,

' ajcathodeand'atleast one other electrode locatedin line;

sists of'a metal support carryingon at leastone su'riace an electron emissive coating containing-one or more 1 metal oxides of lower'functi'on than'the-metal of the support, wherein the oxide coating'of the cathode is activated by a method which comprises connecting the-oxide coatedtcathode to the negative terminal of a direcfcurrent supply connecting 'another electrode of the device to the: positive terminal of the supply, and pas'sin'g an electric current between the cathode and said othe'r 'elcctrode'whilstthe envelope ofthe device is"filled withan ionisablegasfilling at a pressure below the'final-g'as'filling pressure of the device; said 'curren'tand pre'ssurelbing' such that there is obtainedra highly *mobile spai k 'discharge which 'moves 'rapidly fr0in'-p'ointto" f: 1 the surface ofthe oxide coating and r'act'ivates 'th lcoating, the value of the-discharge chrrentbeifig-cohtrfdll A the discharge 'is maintained in said highly mo condition until the acti atiOn-of -th'e coating "byisa'id' method'has been completed-and whereinthis' ipriniary; 'activation'proce'ss isfollowed by a secondaty 'a' 'tivat'ion process in which a series-of shoi't'duration spark-discharges, having peak currents ot-several amp es', are

passed between the cathode and; at least one "othe'iele'c trodeof the device, with the cathode acting as the negathe activationi'offt with "and between the T anode and cathode, and in which the cathode consists ofta metal: support carrying on at a least one surface an 'electronemissive coating containing one or more metal onides "of lower work function than the metal of thesupp'ort, wherein the oxide coating'of the cathode is activated by a primary method which coinprises yconnecting' the cathode to the negative terminal of 'a'di'recft 'current supply, connecting the anode to the positive termi a :of' thefsupply, and passing an electric current betweemthfcathodeand anode whilst the en-' velope'of thedev'ice is fille'd' with an ionisable'rgas'filling s at a pressurehelowthefinal gas .filling pressure of the "d' cu'rrent and ,pressurlefbein'g such that there 7 device; i881 h I is obtained a highly mobile spagle'dischargewhich'moves rapidlyifrompoint to point 'overithe surface of'the coating andactivates the coating, the value of the discharge currentbeing -controlled jduringjthis' activation process 7 so as to lie between limits at which the discharge is maintained in said: highlyjmohile spark condition until s be t' 'r-q i filet P $fi$ 1 ll wqs at qondaj a iva nt w s Whichafseries of hhoiffduration spark discharges, having peak currentscf several amperes isp'asse'd between the eathodeand a d otherelectrodeof thedeyice, with the, cathode. acting as th' i negative j electrode, :WhilSf the 'enf velope 0f 'thedevice refined an ionisable gas filling at approximately "the "fin'alfgas filling pressure fof the device. 7 V: H i f Referencese Citedginthefileof thisipatentt UNlTED STAT-Es "PATENTS; I

2,671,873 Meier M3119, 1954 the device is filled he coating by said' 'primary method dfwherein the primary activation 

